The present invention relates generally to a hydraulic control system for a hydraulic power clutch used for a semi-automatic transmission and particularly to a hydraulic control system of this type which comprises control means for temporarily reducing to a predetermined suitable valve a line or control pressure fed to a hydraulic clutch during disengagement of the clutch.
As is well known in the art, a semi-automatic transmission is provided with a hydraulic power clutch which is combined with a hydraulic torque converter. The clutch is engaged by feeding a line or control pressure outside of a cylinder formed by a clutch cover to cause a piston to force a clutch disk against the clutch cover and is disengaged by feeding the line pressure inside of the cylinder to move the piston away from the clutch disk. The line pressure employed for the engagement and disengagement of the clutch is usually controlled in accordance with the load of the engine. A conventional hydraulic control system for a hydraulic clutch has applied to the clutch a line or control pressure which has a similar value irrespective of the engagement and disengagement of the clutch when the engine load is the same. However, it is desirable that hydraulic fluid pressure used for the disengagement of the clutch is lower than hydraulic fluid pressure used for the engagement of the clutch. This is because the higher the hydraulic fluid pressure is which is employed for the disengagement of the clutch, the greater a force is which acts on a snap ring provided for limiting movement of the piston during disengagement of the clutch and as a result it becomes necessary to increase the strength, size and weight of the snap ring and the clutch cover. This causes an increase in a production cost of the clutch.